Position control system

ABSTRACT

A control system for positioning a machine member from an initial position to a new command position. The member is moved by appropriate motor and control means in cooperation with a delta counter the contents of which continuously represent distance remaining to travel. Initially, data representing the command position are read into temporary storage and compared with data, representing the initial position of the machine member, contained in another (external) counter. A series of pulses, herein designated Delta RCT, increment or decrement the other counter, depending on whether the data in temporary storage is greater than or less than the data in the other counter. Simultaneously, the Delta RCT pulses increment the delta counter away from zero. When the contents of the other counter and temporary storage become equal, generation of the Delta RCT pulses is inhibited; the delta counter then contains how far the machine member must travel to get to the command position. As the machine member moves toward the command position, a second series of pulses (herein designated RCT), each representing motion through an incremental distance, decrement the delta counter toward zero. As the delta counter contents decrease, indicating less distance to go, appropriate speed control signals are provided to slow down the motor. When the delta counter decrements to a predetermined value corresponding to a selected distance from the command position, the motor may be deenergized and the machine member coasts to a halt. Alternatively, when the delta counter reaches the predetermined value, a servosystem may be actuated which drives the member to the command position. Should the member stop at other than the command position, the residual contents of the delta counter will represent the positioning error. The delta counter then is decremented to zero, and the contents of the other counter thereby correspondingly altered so as to represent the actual stop position. Additionally, the system includes storage of zero offset data. This feature has the advantage of resuming operation after an interruption, automatically, without extensive setting up operation or computation.

United States Patent [72] Inventor Robert W. Tripp Tuckahoe, N.Y. [21]Appl. No. 814,670 [22] Filed Apr. 9, 1969 [45] Patented Oct. 12, 1971[73] Assignee lnductosyn Corporation Carson City, Nev.Continuation-impart of application Ser. No. 729,018 May 14, 1968, nowabandoned.

[54] POSITION CONTROL SYSTEM 14 Claims, 18 Drawing Figs. [52] US. Cl318/603, 235/151.l1, 235/92 MP [51] Int. Cl G05b 19/18 [50] Field ofSearch 235/92 (28),151.11;3l8/20.320 [5 6] References Cited UNITEDSTATES PATENTS 3,209,222 9/1965 Holy 318/28 3,223,830 12/1965 Evans..235/92 3,473,098 10/1969 Waller.... 318/162 3,473,100 10/1969 Anger318/28 Primary ExaminerEugene G. Botz Attorney-William E. BeattyABSTRACT: A control system for positioning a machine member from aninitial position to a new command position. The member is moved byappropriate motor and control means in cooperation with a delta counterthe contents of which continuously represent distance remaining totravel.

Initially, data representing the command position are read intotemporary storage and compared with data, representing the initialposition of the machine member, contained in another (external) counter.A series of pulses, herein designated ARCT, increment or decrement theother counter, depending on whether the data in temporary storage isgreater than or less than the data in the other counter. Simultaneously,the ARCT pulses increment the delta counter away from zero. When thecontents of the other counter and temporary storage become equal,generation of the ARCT pulses is inhibited; the delta counter thencontains how far the machine member must travel to get to the commandposition.

As the machine member moves toward the command position, a second seriesof pulses (herein designated RCT), each representing motion through anincremental distance, decrement the delta counter toward zero. As thedelta counter contents decrease, indicating less distance to go,appropriate speed control signals are provided to slow down the motor.When the delta counter decrements to a predetermined value correspondingto a selected distance from the command position, the motor may bedeenergized and the machine member coasts to a halt. Alternatively, whenthe delta counter reaches the predetermined value, a servosystem may beactuated whichdrives the member to the command position.

Should the member stop at other than the command position, the residualcontents of the delta counter will represent the positioning error. Thedelta counter then is decremented to zero, and the contents of the othercounter thereby correspondingly altered so as to represent the actualstop position. Additionally, the system includes storage of zero offsetdata. This feature has the advantage of resuming operation after aninterruption, automatically, without extensive setting up operation orcomputation.

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ATTOZ/VEV POSITION CONTROL SYSTEM RELATED APPLICATIONS The presentapplication is a continuation-in-part of the inventor's copending U.S.application Ser. No. 729,018, filed May 14, 1968, now abandoned,entitled POSITION CON- TROL SYSTEM. The function generator incorporatedhereinbelow is set forth in the inventors copending U.S. applicationSer. No. 645,161, filed June 12, 1967, entitled DIGITAL-TO- ANALOGCONVERTER, now Pat. No. 3,514,775. The position readout apparatusincorporated hereinbelow is set forth in the inventors copending U.S.applications, Ser. No. 739,579 filed May 14, 1968, now abandoned, andSer. No; 809,533, filed Mar. 24,1969 both entitled POSITION MEA- SURINGSYSTEM.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a position control system and, more particularly, to such asystem using an incremental counter in controlling the position of amovable machine member.

2. Description of Prior Art Positioning systems have been constructed ofthe so-called slowdown and stop type where one or more signals are givenin anticipation of the final positioning point to decrease the feed rateappropriately so that the movable member will stop and position mostprecisely. Other systems have been constructed employing a servo drivewhich servos the movable member to the final positioning point.Generally, in either case the final positioning occurs within a cycle ofthe positionmeasuring device used by the system.

A position-measuring device usable by certain systems is described inU.S. Pat. No. 2,799,835 for Position Measuring Transformer by R. W.Tripp et al., issued July 16, 1957, and sold under the trademarkINDUCTOSYN, registered in the US. Patent Ofiice.

The Inductosyn device, whether in a linear or rotary form, senses themovement of a machine based on the inductive coupling between conductorsof the primary and secondary windings separated by a small air space.The displacement of the primary and secondary windings is represented inelectrical degrees. The spacing of three consecutive conductors of thesecondary winding corresponds to a cycle of 360 electrical degrees whichis equivalent, for example, to 0.2 inches. The 0.2 inches would be equalto a cycle of the measuring device.

The unique manner of operation enables a very high accuracy in themeasurement and control of linear or rotary displacement. The usualcoupling and counting problems associated with conventional encoders andoptical scales are eliminated.

The present invention relates to a system for measuring and controllingthe position of a machine having fixed and movable members wherein therelative position of the members is measured and/or controlled byequipment including a position-measuring transformer which has a cycleof small dimension in comparison to the total distance to be measured orthe distance throughout which the position is to be controlled.

It is therefore necessary that a precise measurement be made within thecycle of the position-measuring transformer and also that themeasurement include the larger dimension external to one cycle which maybe many cycles of the position-measuring device.

A further difficulty is that the starting point of measurement on thepart to be measured may not coincide with the fiducial point of themeasuring device. The present invention eliminates these difficulties ina novel and simple manner and provides a unique and simple method toestablish once and for all this difference on any one setup of a part tobe measured. This difference is computed and stored in the measuring andcontrolling system. Heretofore this reconciliation was made by computingthe difference between the programmed position point and the startingpoint of the cycle of the positionmeasuring device adjacent to the samepoint. When a successive measurement is programmed in the same setup anew different dimension is encountered.

The difficulty is avoided in the present invention. In order tocorrelate the portion of the dimension outside of the measuring cycle,it is necessary to know the relationship between the starting point ofthe measurement on the part and the fiducial point of the measuringsystem.

By the present invention only one difference dimension is determined foreach setup and used for all successive programmed positions.

At the time of setup, the dimensional difference between the startingpoint on the part and the fiducial point of the measuring device iscomputed and memorized in the system. Thereafter, as measurement aremade of successive programmed positions, it is not necessary torecompute a difference dimension for each measurement.

The computation and storage of the dimensional difference between thestarting point on the part and the fiducial point of the measuringdevice is described and claimed in the referenced application for aPosition Measuring System filed May 14, 1968, Ser. No. 739,579, nowabandoned, by R. W. Tripp. That application also sets forth and claimsapparatus for measuring and reading out other positions on the part, themeasurements automatically being referenced to the starting point on thepart.

In the program control systems using the slowdown or the servo method,it may be necessary to have signals anticipating the program position tocause the feed rates to change or to cause the servo to function. Thisinvolves computations of position for each program point of which theremay be many.

By the present invention, a how-far-to-go counter is provided. This is acounter which always counts down to the program position. Heretofore,this difiiculty has been overcome in part by providing sequentialprogramming where the operator is required to program each succeedingprogram position from the preceding program position.

By the present invention, sequential programming is not required, thatis, positions may be programmed with respect to a zero referenceposition.

In analog systems, analog offsets have been employed providinganticipatory position signals which are removed as the machine proceedsfrom one slowdown position to another or the final position.

The present invention, being digital in nature, accomplishes the resultby indicating at certain counts of the how-far-togo counter where thesefunctions are to be performed. These functions are performed for eachprogrammed position without disturbing the counter.

One type of digital system for controlling the position of a machinemember is described in U.S. Pat. No. 3,117,263 for Automatic PositionControl Devices, Jan. 7, 1964, by A. T. MacDonald. In that system, amovable member is initially driven in response to the difference betweena coarse positioning command and the actual position of the member untilcoarse and actual position are equal. Subsequently, the member is drivenin response to the difference between a fine positioning command and theactual position of the member until the fine command and actual positionare equal.

The system uses a position measuring transformer of the type describedabove having a plurality of sequential operating cycles. The coarsecommand represents a position equal to an integral number of said cycleswhereas the fine command represents a position within the cyclefollowing the last cycle of the coarse command position.

BRIEF DESCRIPTION OF INVENTION Briefly, the invention comprises a systemfor positioning a movable machine member. The system includes means forcomparing digital data representing a present command position withdigital data representing a prior command position or with a zerocommand position if a prior command has not been executed.

If an inequality is determined, the prior command position in changeduntil the commands are equal. Simultaneously, an incremental counter ischanged an amount equal to the difference. The incremental countercontrols the drive velocity of the movable member.

In one mode of operation, the drive velocity is reduced at preselectedcounts until the stop point is detected, at which time the member ispermitted to coast to the commanded position. Selection of the countsfor reduction of the drive signal is a function of the machinecharacteristics. In different machines, the complete removal of thedrive signal, or selected stop point, may occur at different counts.

The counter is not automatically cleared after the slowdown and stopmode. It may pass through zero and count in the opposite direction ifthe member coasts, or is driven, past the commanded position. As aresult, a new difference command is algebraically summed with theresidual count in the counter and cumulative errors are eliminated.

In another mode of operation, the incremental counter is rapidly countedto zero at a preselected position as determined by the count in thecounter. The count resulting, as the incremental counter is reduced tozero, is stored in an internal counter and used as a fixed commandsignal during the servo mode. The system operates in response to thefixed command signal as a closed loop servosystem and the movable memberis driven the remaining distance to the command position.

If the member is driven past the commanded position and the error signalincreases above a certain magnitude, the movable member is driven in theopposite direction until the preselected point is detected from theopposite direction and the servo mode is again entered. The finalpositioning can be made from either direction.

When the servo mode is entered, the characteristics of the drive meansare alterable to optimize positioning control near the command position.

Display means are provided for indicating the least significant digitsof the position represented by the count in the incremental counter.Inputs to the display can also be used for providing BCD signalsdirectly to other systems such as a computer for automaticallycorrecting for a positioning error. The system also includes means forgenerating a linear control signal proportional to the count in theincremental counter.

The individual parts of the system are designed for a numerical codeparticularly suited for use by the associated part without loss of thecapability for synchronized operation of the complete system. Selectedcounters can be interconnected even though the numerical codes may bedifferent. One of the selected counters is used to control the other asa function of system operations.

Therefore, it is an object of this invention to provide a simple systemwhich has the capability of controlling the position of the machine andalso reading out any position of the machine at command.

Another object of the invention is to provide an improved digitalpositioning system for algebraically summing the data representing thedisplacement of a machine member from a prior command position with datarepresenting the difference between the prior command and a presentcommand and for driving the member to a position represented by the sum.

A further object of the invention is to store the difference between acommand position and the actual position of a machine member and toreduce the stored difference to zero as a function of the machineposition when driving the member in a servo mode.

Still, another object of this invention is to provide a means fordisplaying the position of the machine on command.

A still further object of the invention is to provide the capability forinterconnecting selected counters of the system and for controlling onecounter with the other as a function of system operations.

Still a further object of the invention is to provide a position controlsystem having linear servocontrol characteristics.

A further object of this invention is to provide a system foralgebraically summing the difference in consecutive, command positionswith the displacement of the actual machine position from prior commandposition for eliminating cumulative errors.

Another object of this invention is to provide a positioning systemhaving a display of position error.

Still another object of this invention is to provide an improvedposition control system not requiring incremental programmmg.

BRIEF DESCRIPTION OF DRAWINGS FIGS. la and 1b represent a functionalblock diagram of one embodiment of the invention.

FIG. 2 is a schematic diagram of one embodiment of an incrementalcounter including detection logic.

FIG. 3 illustrates an example of velocity reduction.

FIG. 4 represents an embodiment of one decade of a reversible counterusable in the system.

FIG. 5 is a vectorial illustration of the relationship between pulsesused in generating signals representing trigonometric functions.

FIG. 6 represents one embodiment of a function generator usable in thesystem.

FIG. 7 represents one embodiment of internal control logic forcontrolling the internal counter and function generator during systemoperations.

FIG. 8 represents one embodiment of external control logic forcontrolling the sequence of operations of the system during the setupand readout modes.

FIG. 9 is a schematic diagram of logic for controlling the countingdirection of the incremental, or delta, counter.

FIG. I0 is a illustration of one comparison decade between one decade ofthe temporary storage device and one decade of the external counter.

FIG. 1 l is a schematic diagram of a circuit for switching into a servomode as a function of the count in the incremental counter.

FIG. 12 illustrates a switch usable in the data distribution fortransferring data into the temporary storage device.

FIG. 13 is an illustration of a switch usable in the data distributionfor controlling the mode of system operations.

FIG. 14 shows a modification of the control system for producing alinear error signal from the incremental counter.

FIG. 15 is a diagram of the function generator waveforms.

FIG. 16 shows a modification to the FIG. 2 logic for precisely selectinga final point.

FIG. 17 diagrammatically illustrates typical setup, readout andpositioning operations carried out by the inventive position controlsystem.

DESCRIPTION OF PREFERRED EMBODIMENT FIGS. la and lb show a block diagramembodiment of a point-to-point positioning system for controlling thepositioning of a movable member of the machine relative to a fixed partof the machine along one axis. The FIG. la portion, in-

. cluding the external counter of FIG. lb, is described in thereferenced patent application for a Position Measuring System, nowabandoned.

Although machine members are not specifically illustrated by thedrawings, the description assumes that the machine members areillustrated by the representations for the members of theposition-measuring device. Additional control systems may be providedfor each axis of a multiaxis machine.

The system includes clock generator 1 for generating a signal (CK) at,for example, a frequency of 4 megacycles which is used as a countingsignal by reference counter 2 and as a timing signal by other parts ofthe system as described herein. The signal may be in the form ofrectangular pulses which have a repetition rate equal to 4 mc. Althoughthe output from the clock generator 1 is described as a signal, itshould be understood to mean an alternating voltage level having thefrequency indicated.

The clock signal may be produced by capacitor charge and discharge timethrough a resistor for controlling a gate output or by other circuitswell known in the art.

Reference counter 2, which includes three decade counters 230, 23k and232, furnishes signals proportional to the count in each decade on 1, 2,4, and 8, and T, E, f, and a binary coded decimal (BCD) conductors forthe units (U), tens (T), and hundreds (H) counts. The thousands Th stage233 which may comprise for example, a J K flip-flop provides binaryoutputs at l and l for example, each time 1,000 clock pulses arereceived. The flip-flop receives the carry output from the hundredsdecade 232. The count signals from each decade are compared incomparator 3 with corresponding signals from internal counter 4. Thesignals from internal counter 4 are proportional to the count in theinternal counter 4. The counting signal for the internal counter 4, inthe form of counter toggling (RCT) pulses, is generated by internallogic 5 as a function of error signal, e.

The 4 me. signal is received at input 6 of the reference counter 2 andis divided into signals having frequencies of, for example, 2 kc. and200 kc. The 2 kc. signal provides a basic counting rate for the systemand the 200 kc. signal provides an increased counting rate for thesystem as subsequently described. For the embodiment described, a cycleof the reference counter 2 is divided into 2,000 equal parts i.e., thereference counter 2 counts 2,000 clock pulses each cycle. Other cyclesand divisions are also suitable.

The 2 kc. and 200 kc. signals are connected to internal control logic 5by conductors 7 and 8, respectively. The examples indicated above areused throughout the description of the embodiment of FIGS. 1a and lb. Itshould be obvious that the system is not limited to the particularexamples selected.

Similarly, although a combination of decimal, binary and binary-codeddecimal numerical systems with corresponding circuitry is used in thisdescription, it should be understood that other numerical systems arewithin the scope of the invention. For example, a straight binarynumerical code and appropriate circuitry could also be used.

For the particular embodiment described herein, the internal counter 4has a counting capacity equivalent to a cycle of the position-measuringdevice 11 so that its count represents the position of the movablemachine member (illustrated as part of reference number 12) relative tothe fixed machine member (illustrated as part of reference numeral 13)in a cycle of the measuring device 11. For example, if the measuringdevice 11 has a cycle of 0.2 inches, a count of one, assuming a 2,000count cycle, would be equivalent to 0.0001 inches of movement.

Internal counter 4 includes three reversible decade counters 234, 235and 236 for counting in binary-coded decimal and a binary stage 237 forcounting straight binary. Each of the decade counters 234, 235, and 236generate A, B, C, D and A, B,C, 5 outputs representing units, tens andhundreds of RCT pulses counted b intemal counter 4. The binary stage 237generates A and A outputs representing thousands of RCT pulses countedby the internal counter 4.

One example of a typical decade counter which could be used as a decadeof the internal counter 4 is shown in FIG. 4.

The internal counter 4 receives CL, signals from external control logic9 for setting the output counts to zero and a U/D, signal from theexternal control logic 9 for controlling the counting direction of theinternal counter 4.

Comparator 3, which includes four comparison stages 296, 297, 298 and299, compares counting signals from reference counter 2 with countingsignals from internal counter 4. In one embodiment, the comparisonstages 296, 297, 298 and 299 are implemented for example by NAND logicgates having inputs from internal counter 4 and reference counter 2. Thecomparator 3 may also include, for example, logic gates for convertingthe internal count from internal counter 4 into a nines complement priorto being compared with the reference count from reference counter 2.

Whenever the count in the internal counter 4 coincides with the count inthe reference counter 2, positive coincidence signals .+TU (tens andunits), +Th (thousands) and +H (hundreds) are generated as input signalsto function generator 10. The TU conductors are connected together as amatter of design since the tens and units coincidence signals formcommon inputs to a gate in the function generator 10 as shown in FIG. 6.

Whenever the nines complement of the count in internal counter 4coincides with the reference count, negative coincidence signals, TU(tens and units), Th (thousands), and H (hundreds) are generated asinput signals to function generator 10. The TU conductors are connectedtogether for reasons indicated above.

Overall positive and negative coincidence are referred to herein ascoincidences in the +11 and n channels, respectively. As a result ofusing a nines complement, the n coincidence signal is early by one clockperiod. However, a flipflop can easily be used for delaying the n signalby one period for correction of the data as shown in FIG. 6.

Function generator 10 uses the coincidence signals in the .+n and nchannels to develop pulse width signals representing trigonometricfunctions for the input windings (part of movable member 12) of positionmeasuring device 11 which comprises a position measuring transformer ofthe type described in U.S. Pat. No. 2,799,835, previously referenced. itis pointed out that although a particular type of position-measuringdevice is used in describing a specific system embodiment, other devicessuch as resolvers could be used.

The pulse width of the signals from the function generator 10 isdetermined by the spacing of the coincidence signals, n and .+n, from areference position. For the particular embodiment shown, the signalsrepresent sine and cosine trigonometric functions indicated by S and Con the input conductors to position measuring device 11.

A more detailed analysis of the trigonometric relationship between thecoincidence signals can be found by referring to patent application Ser.No. 645,161 for a Digital-to-Analog Converter by R. W. Tripp filed June12, 1967, now US. Pat. No. 3,514,775.

The device includes movable member 12 positionable, for example, alongan X axis and fixed to a movable member of the machine. Shaft 196 isshown connected to movable member 12 for positioning the movable member12 and the movable machine member included in the representation of themovable member 12. The movable member includes polyphase windings suchas a pair of multipolar primary windings phase shifted in space. Fixedmember 13 of the device includes a continuous conductor forming amultipolar secondary winding for the device. The member is connected toa fixed machine member such as the frame of the machine.

The movable and fixed members 12 and 13, respectively, of theposition-measuring device ll may be either linear or rotary in formdepending on the particular application of the system.

As is well known, in such a position-measuring device, the position ofthe movable member, with respect to the fixed member, can be representedby the relative displacement of the secondary winding with respect tothe pair of primary windings. The displacement is represented as anangle measured in electrical degrees. It should be understood that thespacing of three consecutive conductors of the secondary windingcorresponds to a cycle of 360 electrical degrees which is equivalent,for example, to 0.2 inches. For the example given, the device would passthrough a cycle every 0.2 inches.

Sine reverse switch has a and a position for reversing the polarity ofthe sine signal into the position measuring device 11. By reversing theswitch position, a positive number may be represented by either left orright machine motion relative to a reference on, for example, aworkpiece.

The minus input for reversing the polarity of the sine is connected tothe function generator 10 when the arm 15 of the switch 14 is down. Arm15 of the switch is shown connected to electrical ground for providingthe relative low, or minus, input.

v the positiontrneasuring device 11 and filter 17 for amplifying theerror signal from the position-measuring device 11. All harmonics exceptthe basic frequency signal being used by the system, are filtered byfilter 17. Filters and preamplifiers are well known in the art and forthat reason details are not included. The error signal from thepreamplifier 16 is changed into a sinusoidal signal having a magnitudeas a function of the difference between the command position and theactual position of the movable member 12 in a cycle of the measuringdevice.

The output signal from the filter 17 is processed through phase detector19 for generating a DC error signal, e. The error signal, e, has apolarity (sign) as a function of the direction of relative movementbetween the movable and fixed machine members. Phase detectors andassociated circuitry for generating DC voltages are well known in theart. The error signal, e, is used in generating RCT pulses and anup/down control signal (U/D,,). In addition, as shown in FIG. lb, theerror signal, e, is used for driving a servomotor 195 during the servomode.

The output signal from filter 17 is also processed through full 'waverectifier 18 for generating frequency control signal, Es. The Es signalis also used as shown in FIG. lb, for switching the system out of theservo mode under certain conditions. The signal has a DC voltage levelas a function of the amplitude of the output signal from filter l7 andis used as described in FIG. 7 in changing the counting rate from onefrequency to another as a function of its magnitude. F ull-waverectifiers and associated circuitry are also well known in the art.

Internal control logic generates RCT counting pulses as a function ofsignals e, Es and the clock, CK. RCT pulses are inhibited as a functionof changes in the sign of the error signal, e, for preventingoscillation of the least significant digit of the display 21. An inhibitsignal (Inh) is generated for use by data distributor and mode control33 shown in FIG. 1b.

Generation of the RCT pulses is synchronized within the internal controllogic as a function of coincidence between units comparison signals, ICTand ICV, from the units decade 230 of reference counter 2 and the unitsdecade 234 of internal counter 4. Synchronization is necessary, asdescribed in detail with FIG. 7, to prevent counting errors.

Internal control logic 5 also generates a U/D, signal for controllingthe counting direction of internal counter 4 and external counter 20through external control logic 9. The logic state of the U/D, signal andtherefore the counting direction is controlled by UD, and SU, signalsgenerated by the external control logic 9 during a setup mode and by thesign of error signal, e, indicating direction of machine motion during anormal readout operation.

The frequency of the RCT pulses can be changed from 2 kc to 200 kc as afunction of the mode of operation or the magnitude of the error signalas indicated by Es.

A 2 me. counting signal is generated for use by external control logic 9as described in connection with FIG. 8.

External control logic 9 includes a binary counter 134 counting at a 2me. rate (see FIG. 8). The count is decoded for controlling the sequenceof system operations during the setup and readout modes. The externalcontrol logic 9 provides an up/down (U/D,) signal for controlling thecounting direction of external counter 20 (FIG. lb) as a function of thedirection of movement of the movable machine member represented by thelogic state of the UID, signal. A signal is provided to the display 21for displaying the sign of the number contained in the external counter20. The display 21 is slaved to the external counter 20.

A reset signal (RS) is generated by the external control logic 9 toclear the external counter 20 and a CL, signal is generated for clearingthe internal counter 4.

The external control logic is described in more detail in connectionwith FIG. 8.

Mode switch 24 includes readout (R0) and setup (SU) positions forcontrolling the mode of system operation. Conductors labeled R0 and SUcontrol the external control logic 9 as a function of the position ofthe arm 238 of the mode switch 24 and other conditions describedsubsequently. The arm 238 is connected to electrical ground forproviding a relatively low input to the external control logic 9.

Reset switch 25 is connected to the external control logic 9 forresetting external counter 20 and display 21 to zero when it is desiredto establish a new reference position and at the start of the setupmode. The switch 25 may be a pushbutton mounted on the front panel forapplying an electrical potential to the external counter 20 for settingthe counter 20 to logic zero. The input to the external control logic 9from the reset switch 25 is designated as CL.

External counter 20, shown in FIG. lb, comprises six reversible decadecounters 239 through 244 of the type shown and described in detail inconnection with FIG. 4. The l, 2, 4 and 8 (BCD) output conductors fromeach decade counter are connected to the corresponding decades Q throughof display 21. One line is used to represent the output cbnductors fromthe four stages of each decade. It should'be understood that fourconductors would be connected between each decade and a displayindicator. The single line is used for convenience.

The output conductors of external counter 20 are also connected to therespective l, 2, 4 and 8 terminals of storage device 22 which stores thepart starting number, or position. The conductors connected to the fourleast significant digits of the external counter 20 are connected tostorage device 23 which stores the displacement, or offset, numberdetermined during the setup mode as described in detail subsequently.

In addition, as described in FIG. lb, the BCD bipolar outputs A, B, C, Dand A, E, C, and D from the counter 20 are connected to comparator 35.

Coincidence conductors, PS and OS, for storage devices 22 and 23,respectively, provide external control logic 9 with signals indicatingcoincidence between the count in the external counter 20 and the storeddigits at appropriate times dur-.

ing the setup mode.

The offset storage device 23 may comprise four BCD thumbwheel switchesconnected to the least significant decades of the counter 20 (and theleast significant digits of the display 21). The part start storagedevice 22 may comprise six thumbwheel switches and a sign switchconnected to appropriate decades of counter 20 and the external controllogic 9 (and the display 21). Other storage devices including relays,solid-state devices, computer storage, tape, etc. may also be usedwithin the scope of the invention.

Display 21 may comprise six decimal-indicating cold cathode tubes and asign indicating cold cathode tube including appropriate decoding anddrive circuits for converting the BCD outputs from the counter 20 intodecimal indications for the display 21. Other devices such as theexamples given in connection with storage devices 22 and 23 could alsobe used.

In the preferred embodiment, the mode and reset switches, 24, 25,thumbwheel storage devices, 22, 23, and display 21 are mounted on thefront panel.

As indicated above, the bipolar outputs (A, A, etc.) from externalcounter 20 are one set of inputs to comparator 35. The other set ofinputs to the comparator 35 are provided by temporary storage device 34.

An example of one decade of comparator 35 and one decade of temporarystorage device 34 are shown and described in detail in connection withFIG. 9. The temporary storage device 34 includes a stage 245 for storingthe sign of its data. The sign of the data in the external counter 20 isprovided as a plus, minus output from the external control logic 9, tothe comparator state 246.

Data in the temporary storage device 34, representing a present commandposition, i.e., the position to which a movable machine member is beingcommanded to assume, is compared with data in the external counter 20which represents the previous command position, i.e., the position towhich a movable machine member was previously commanded to assume. Ifthe data is equal, an equal signal, E0, is generated.

if the data in the temporary storage device 34 is less than the data inthe external counter 20, a less signal is generated from the comparator35. The more/less signals are generated on the M/L line. The E and M/Llines are connected to data distributor 33 which directs counting pulsesinto external counter and into the incremental, or delta counter 36 asdescribed subsequently. A schematic embodiment of a delta counter 36 isshown in FIG. 2.

The counting direction of the delta counter 36 is controlled by up/down(U/D A) control logic 32 which generates an up/down counting controlsignal U/D A. A specific embodiment of the up/down (U/DA) control logic32 is shown in FIG. 9. The up/down control logic 32 receives RCT pulses,a CL, signal, M/L and U/D signals. Those signals set the output, line43, on the up/down control logic 32 for controlling the countingdirection of the delta counter 36.

RCT and ARCT pulses are directed by the data distributor 33 to theexternal and delta counters (20, 36) as described subsequently. ARCTpulses load the delta counter 36 with the computed difference betweenthe data contained in the temporary storage device 34 and the externalcounter 20. RCT pulses reduce the count in the delta counter 36 towardszero as the movable machine member is positioned from one point toanother on a workpiece (not shown).

The delta counter 36 receives an lN signal on line 277 as described inconnection with FIGS. 2 and 13.

Position error display device 31 is connected to the delta counter 36 tovisually display the least significant digits of data in the deltacounter 36. After a machine has been positioned, the display 31indicates the position error. The display 31 which may comprise a twostage counter, is loaded by ARCT pulses during the transfer of pulses tothe external counter 20 and is unloaded by RCT pulses during thepositioning of operation. A plus, minus indicator, 2:, controlled byup/down delta control (U/DA) logic 32 is also shown for displaying thesign of data in delta counter 36.

As the ARCT pulses load the delta counter 36, the two digits are alsoloaded. The carry from the most significant digit is dropped.

The display 31, therefore, counts and displays any number between 00 and99. This numerical range would represent a linear range of 0.0000 to0.0099 inches. When loading operation is complete, the display 31 willcontain the least two significant decimal digits of the number in thedelta counter 36.

Conversely, as the RCT pulses unload the counter 36 during thepositioning cycle, the display 31 will also be unloaded. If the finalmachine position is exactly as commanded, both the delta counter 36 anderror display 31 will be unloaded to zero. if, however, the machine (notshown in detail) does not position as commanded, a numerical residue,equivalent to the error, will exist and appear at the display 31. Alight may be added to indicate if the error is in excess of the digitsdisplayed.

A serial detection circuit (not shown) and a selector switch (not shown)may be added for selecting an output from the delta counter 36corresponding to the desired stop point. The operator could optimize thelocation of the final stop point and minimize any error which may existafter the machine has coasted to a stop.

Operator selection of whether the inventive position control system isto function in the SETUP, READOUT or POSI- TIONING mode is achieved bymeans of mode select switch 51 (P10. lb). Linkage 51 connects modeselect switch 51 to arm 238 of switch 24 so as to insure proper SU or R0input to external control logic 9 during setup and readout operation,respectively. in the various positioning modes described hereinbelow,switch 51 cooperates with circuitry in data distributor and mode control33 to advance switch 209 (FIG. 11) to the appropriate setting.

Data distributor and mode control 33 controls the distribution of datafrom one of a plurality of sources including dial switches 29 and tapereader 28 to temporary storage 34 and to machine control device 27 or tosystems for controlling other axes of a multiaxis machine. In a simpleembodiment the data distributor 33 may be implemented by manual switchesas shown in FIGS. 11 and 12. In a more complex embodiment, counters andlogic gates could be used. The data distributor 33 receives a strobesignal on line 192 from reference counter 2 for synchronizing thetransfer of data into storage device 34.

information stored on perforated tape (not shown) in the tape reader 28would be coded to address one of the plurality of axes, indicate thecoordinate position to which machine member (reference numeral 12) wasto be driven along-the addressed axis, or command an auxiliaryoperation. The data distributor 33 decodes the address and provides theappropriate command signals to the addressed axis.

Depending on a particular application, a tape reader 28 could be amechanical or high-speed optical device for reading multitrack datacontained in consecutive rows on perforated tape (not shown). Details ontape readers, usable within the scope of the invention are known topersons skilled in the art and are not included.

Machine control device 27 may, for example, be composed of relays,gating logic, etc., for providing control signals to motor controldevice 26 and signals to other parts of the machine (not shown)representing auxiliary commands. The machine control device 27 alsoreceives feedback signals indicating the response of the machine to theauxiliary commands. SP and e input signals to machine control device 27are processed through relay 229 as a function of the systemoperatingmode. For example, the relay 229 may be a double pole double throwdevice so that the error signal, e, can be switched from the internalcontrol logic 5 to the machine control device 27 for driving the motor195 during the servo mode. For slowdown and stop operation, the motor195 is controlled by slow down and stop signals (SP). Relay controldevice 227 generates a signal to relay driver 228 for actuating therelay 229 when the detected servo count is greater than the magnitude ofthe frequency control signal, Es. Es has a magnitude proportional to themachine position error.

It should be pointed out that in a practical embodiment, the devicesdescribed herein as comprising the system would be interconnected forone mode or the other so that the necessity for switching from one modeto another would never arise. However, if desired and as describedherein, a switching device (relay 229) can be used to switch from onemode to another without difi'iculty.

Data selector 191 determines whether or not data to be distributed isreceived from the tape reader 28, a computer (not shown) or the dialinput switches 29. it can be set in either a tape (automatic orcontinuous) or a dial mode. input data may also be accepted from othersources.

Read switch 199 may comprise a pushbutton switch for enabling the datadistributor 33 to begin reading the data from the selected source. Theswitches 199 and 191 may be used with embodiments other than theembodiment shown in FIGS. 1a and 1b.

Dial input switches 29 may comprise six manually operated switches 253through 258 which can be rotated to a selected position. A switch 292indicates the sign of the data. The decimal numbers set into switches253 through 258 are converted into BCD numbers and sequentiallytransferred into the temporary storage 34 through data distributor 33upon receipt of strobe signals on lines 247 through 252 which also goesto storage device 34. The rotating arm (shown in switch block) of eachswitch of input switches 29 is connected to the strobe lines 247 through252 from data distributor 33 so that each switch is sequentiallyactuated for transmitting data to the storage device 34 via line 276through the data distributor 33. The 1, 2, 4 and 8 conductors for eachswitch are common to l, 2, 4, and 8 conductors from the other switches.Similar dial switches would be provided for each axis of the machine.

Motor control device 26 receives signals from'the machine control device27 as a function of the operating mode of the system. The motor controldevice 26 may be comprised of clutches and brakes, servo electronics,hydraulic controls, etc., for controlling the dynamic characteristics ofmotor 195 in response to the signals. The motor is connected by shaft196 to the movable member 12 of the position-measuring device 11 shownin FIG. la.

Tachometer 194 provides velocity feedback to the motor control device 26during certain phases of the system operation.

FIG. 2 illustrates a functional block diagram of delta counter 36including logic 119. The counter comprises 20 binary stages which mayfor example be implemented by flip flops and interconnecting logicgates. The stages 2 through 2" are connected in cascade so that theoutput from one stage provides an input to the next stage.

The first, or 2, stage receives input signals ARCT and RCT for loadingand unloading the counter 36 respectively. ARCT pulses are provided bythe distributor 33 for loading the counter 36 to the difference betweena new command position and the prior command position. Any residualcount in the counter 36 is algebraically added to that difference. RCTpulses are produced by the internal control logic as the machine isdriven towards the new command position for unloading the counter 36.

Detection logic 119 is implemented by series connected NOR gates. Thelogic one output from each stage of the counter 36 provides an input toa NOR gate, for example, NOR-gate 259 associated with the I6th stage. Ifboth inputs to the NOR-gate 259 are low, the output is high. Anotherinput to the NOR-gate 259 is received from a NOR gate, for exam ple,NOR-gate 260 interconnected between NOR gates associated with eachstage. A high output is inverted by the interconnecting NOR gate.

As a result, if both inputs to a NOR gate are low, the output will behigh, and the output from the interconnecting NOR gate will be low. Ifone of the inputs to the NOR gate is high, the output will be low andthe remaining gates of the detection logic 119 are inhibited.

The output from the last NOR-gate 261 of the series combination providesan indication of a zero condition (ZD) for the counter 36. The zerodetect signal (ZD) may result from a clear (CL,,) signal generated bythe internal control logic 5 at the beginning of system operations or asa'result of being counted to zero during the loading and unloadingprocess. The zero condition is used as described in connection with FIG.9 for controlling the counting direction of a delta counter 36.

Logic gating combinations including NAND-gate 121, AND-gate 122,NAND-gate 197, AND-gate 198, NAND-gate 188, and AND-gate 189, provideoutput signals SP1, SP2 and SP3, respectively, for slowing down andstopping the machine. These logic circuits detect the counts at whichthe machine is to be slowed down and stopped, as shown graphically inFIG. 3 for a specific example. The last point could be used to switchthe system into a servo mode.

The delta counter 36 is described more specifically in con nection withFIGS. 9 through 11 and generally in connection with FIG. 3. FIG. 3 isused subsequently herein in describing the operation of the system.

FIG. 4 illustrates one embodiment of a reversible BCD counter 262 whichcan be used as a decade of the counter described in connection withFIG. 1. For example, six such counters would be used in implementingexternal counter and three counters could be used with o ne flip-flopfor implementing internal counter 4. The Q and Q outputs frgn the J Kflip-flops 52 through 55 represent BCD bits 1, l, 2, 2, 4, 4, 8, 8,respectively.

NAND-gates 56 through 59 control the flip-flops 52 through 55 when thedecade 262 is counting up and NAND- gates 60 through 63 control theflip-flops 52 through 55 when the decade 262 is counting down. The U/Dinputs to NAND- gate 64 determine whether or not the decade 262 is setto count up. In that case, the low output from gate 64 is inverted byNand gate 68 to set the decade 262 to count up. In the event the inputto gate 64 is low, the output is high and the decade is set to countdown.

Each flip-flop (52 through 55) can be cleared to zero by RS signalgenerated by reset switch 25 if the decade 262 is part of externalcounter 20 or by a clear signal, CL, if the decade 262 is part ofinternal counter 4.

Input pulses to the decade 262 are received on conductor 69 designatedas RCT or CRY IN. The decade 262 could be the first stage of eithercounter 20 or counter 4 and therefore i be receiving RCT pulses or itcould be a subsequent stage of one of the counters (20, 4) and thereforebe receiving carry (CRY) pulses.

NAND-gates 70, 71 and 72 provide the proper output voltage level as toswitch, or toggle, the flip-flops 53 through 55 of the decade from onestate to another. NAND-gate 73 provides a carry (CARRY OUT) to the nextdecade (not shown).

For the particular embodiment shown, gate 76 has been added to determinewhen the decade 262 has a zero count. The zero detect capability is usedby external control logic 9 to determine the setting of the U/D, signal.Additional details are described and shown in connection with FIG. 8 forthe external control logic 9.

Assuming the decade 262 has been cleared and U/D is high, a pulsereceived at the input of flip-flop 52 sets the O output high. No otherflip-flop would be set since NAND-gates 56 through 59 would be inhibitedby the zero setting of the flipflops 53 through 55, prior to receipt ofthe first pulse.

Upon receipt of the next input pulse, the Q output from flipfiop 2changes states from high to low. The output from gate 56 is high,thereby setting the output from gate 70 low upon receipt of the nextclock pulse for setting the 0 output from flip-flop 53 high.

The counting sequence is continued until the decade 262 contains a countof 9, at which time the output from NAND- gate 59 is set low. The nextcount pulse then sets the Q output of flip-flop 55 low and produces acarry output from NAND- gate 73 into the next decade (not shown).

When all the 6 outputs are high, thereby indicating a zero condition(ZD), NAND-gate 74 is set low and NAND-gate 75 is set high. When a zerocondition is detected for a preceding decade (not shown), as indicatedby a high signal on the ZD IN conductor, the output of AND-gate 76 to asucceeding decade (not shown) is high.

FIG. 5 is a vectorial representation of coincidence signals, or pulses,+n and -n generated by the comparator circuits of the system.

The circle (360) represents a cycle of the reference counter beginningat reference position 0". For the assumed example, the circle is dividedinto 2,000 equal intervals so that each interval, or count, isequivalent to an angle measured from the reference position. Forexample, a count of l in the internal counter 4 would be equivalent toan angle of 0. 1 8, as the count increases, the angle represented by thecount increases.

The pulses represented by the vectors are equally spaced on both sidesof the reference to eliminate errors caused by undesirable phase shiftsin the system. Phase shifts in the system cause both pulses to beshifted in the same direction relative to the reference. As a result,the pulse separation and, therefore, the command position remains thesame. Addition methods for producing sine/cosine analog waveforms usableby the system as alternatives to the method described and shown, can beseen by referring to U.S. application Ser. No. 645,161 for aDigital-to-Analog Converter by Robert W. Tripp, filed on June 12, 1967now U.S. Pat. No. 3,514,775.

Other signals, or pulses represented by vectors n+500,

generated by the function generator 10. By properly selecting pulse,pulse width analog signals can be generated which representtrigonometric functions in various quadrants of the circle.

1. In a position control system having temporary storage means forstoring a command position to which a machine member is to be moved, afirst counter, the contents of said first counter indicating the presentposition of said member, and motor means for moving said member, theimprovement comprising: a reversible counter, means for loading saidreversible counter with a number equal to the algebraic differencebetween the contents of said temporary storage means and the contents ofsaid first counter, said number representing the distance and directiOnsaid member must be moved to reach said command position, control meansfor controlling operation of said motor means in response to thecontents of said reversible counter, said means for loading comprising acomparator for determining whether the contents of said first counter ismore than, less than, or equal to the contents of said temporary storagemeans, means for decrementing or incrementing said first counter if thecontents thereof are, respectively, more than or less than the contentsof said temporary storage means, said decrementing or incrementingterminating when the contents of said first counter and said temporarystorage means become equal, means for simultaneously incrementing saidreversible counter by a like amount, means for changing the sign of saidreversible counter to indicate whether said first counter is incrementedor decremented. means for decrementing said reversible counter as saidmotor means moves said member toward said command position, and meansfor terminating said motion when said member reaches said commandposition, the residual contents of said reversible counter when saidmember stops representing any positioning error.
 2. A position controlsystem according to claim 1 further comprising means for decrementingsaid reversible counter to zero subsequent to said termination of motionand for accordingly changing the contents of said first counter torepresent the actual position at which said member stopped.
 3. In aposition control system having temporary storage means for storing acommand position to which a machine member is to be moved, a firstcounter, the initial contents of said first counter indicating thepresent position of said member and motor means for moving said member,the improvement comprising: a reversible counter, a comparator fordetermining whether the contents of said first counter is more than,less than, or equal to the contents of said temporary storage means,means responsive to said comparator for providing count pulses to bothsaid first counter and said reversible counter when the contents of saidfirst counter is unequal to the contents of said temporary storagemeans, said count pulses decrementing or incrementing said first counterif the contents of said first counter are, respectively, more than orless than the contents of said temporary storage means, said countpulses also incrementing said reversible counter from zero, said countpulses terminating when the contents of said first counter and saidtemporary storage means become equal, whereby the resultant contents ofsaid reversible counter equal the difference between the contents ofsaid temporary storage means and the initial contents of said firstcounter, said resultant contents representing the distance said membermust be moved to reach said command position, control means forcontrolling operation of said motor means in response to the contents ofsaid reversible counter, and means for causing said count pulses todecrement said reversible counter in response to error current derivedfrom relative displacement of a first member associated with the movable4. In a position control system having temporary storage means forstoring a command position to which a machine member is to be moved, afirst counter, the initial contents of said first counter indicating thepresent position of said member, and motor means for moving said member,the improvement comprising: a reversible counter, a comparator fordetermining whether the contents of said first counter is more than,less than, or equal to the contents of said temporary storage means,means responsive to said comparator for providing count pulses to bothsaid first counter and said reversible counter when the contents of saidfirst counter is unequal to the contents of said temporary storagemeans, said count pulses decrementing or incrementing said first counterif the contents of said first counter arE, respectively, more than orless than the contents of said temporary storage means, said countpulses also incrementing said reversible counter from zero, said countpulses terminating when the contents of said first counter and saidtemporary storage means become equal, whereby the resultant contents ofsaid reversible counter equal the difference between the contents ofsaid temporary storage means and the initial contents of said firstcounter, said resultant contents representing the distance said membermust be moved to reach said command position. control means forcontrolling operation of said motor means in response to the contents ofsaid reversible counter. means for storing a sign indicative of whethersaid first counter has been decremented or incremented, and wherein saidcontrol means causes said motor means to move in a direction determinedby said sign, said motor movement causing said member to advance towardsaid command position, means for decrementing said reversible countertoward zero as said member moves toward said command position, saidmeans for decrementing comprising means for generating counter togglepulses in response to motion of said machine member, each of saidcounter toggle pulses representing movement by said member of anincremental distance, and means for causing said count pulses todecrement said reversible counter, said control means causing said motorto operate at a speed determined by the magnitude of the contents ofsaid reversible counter, logic means cooperating with said reversiblecounter for producing speed control signals responsive to the magnitudeof the contents of said reversible counter, said control means beingresponsive to said speed control signals, said motor control meanscausing said motor to run at progressively slower speeds at decreasingvalues of the magnitude of the contents of said reversible counter,occurrence of the final speed control signal causing deenergization ofmotor means, said final speed control signal being selected so that saidmember coasts to said command position subsequent to deenergization ofsaid motor means, and means for causing said count pulses to decrementsaid reversible counter in response to error current derived fromrelative displacement of a first member associated with the movablemachine member and movable relative to a second member of a positionmeasuring transformer having cyclically spaced zero positions.
 5. Aposition control system according to claim 4 wherein the residualcontents of said reversible counter represent the magnitude of anypositioning error when said member coasts to a halt.
 6. A positioncontrol system according to claim 5 further comprising means forchanging said stored sign if said member coasts beyond said commandposition, said changed sign indicating an overshoot.
 7. A positioncontrol system according to claim 6 further comprising means fordecrementing said reversible counter to zero and for simultaneouslyaccordingly incrementing or decrementing said first counter, dependingon said stored sign, whereby the contents of said first counterrepresent the actual position at which said member halted.
 8. A positioncontrol apparatus including a system for measuring and controlling theposition of a first member movable relative to a second member employinga position-measuring device having cyclically spaced zero positions, amotor for driving said first member, and means providing an error signaldepending on relative displacement of said members, said systemcomprising: an internal counter controlled by said position-measuringdevice, the contents of said counter indicating the relative position ofsaid first member between two adjacent-spaced zero positions, means forstoring a part-start position representing the distance of said firstmember from a reference not necessarily corresponding with one of saidspaced zero positions, an external counter having a grEater number ofdigits than said internal counter, a control logic for presetting saidexternal counter to said part-start position during a setup mode and forslaving said preset external counter to said internal counter during areadout mode, whereby during said readout mode said external counteroperates starting from said preset part-start position as said internalcounter operates starting from said relative position, switching meansfor selecting either a readout mode or a positioning mode, said readoutmode including means for slaving said external counter to said internalcounter, said error signal being operative to supply digital pulses tosaid internal counter and to said external counter, each of said pulsescorresponding to motion of an incremental distance, to increment ordecrement both of said counters depending upon the direction of motionof said movable machine member, said movable machine member beingmovable to a new position where said pulses will increment both of saidcounters, whereby the contents of the internal counter represent the newposition with respect to an adjacent sine zero and the contents of saidexternal counter represent the actual distance of the new position froma reference zero, a delta counter controlling said motor, means forderiving other pulses from said digital pulses for loading said deltacounter depending on a comparison of present and new positions of saidfirst member, and means employing said digital pulses for unloading saiddelta counter.
 9. Apparatus according to claim 8, said positioning modeincluding a slowdown and stop-positioning mode wherein said externalcounter initially contains a prior dimension with respect to a part zeroand the contents of said delta counter being zero, means for supplyingcommand data of a new position, a temporary storage for data of said newposition, a comparator for comparing the contents of said temporarystorage with the contents of said external counter, means responsive tothe output of said comparator for supplying said other pulses to saiddelta counter and to said external counter thereby causing the contentsof said external counter to approach the value of the contents of saidtemporary storage and simultaneously increment said delta counter, meansfor inhibiting generation of said other pulses when said comparatorindicates that the contents of said temporary storage and said externalcounter are equal at which time the contents of said external counterrepresent the new command position and the contents of said deltacounter represent how far said machine member must be moved to reach thenew command position, and means for initiating actual positioning ofsaid first member upon said inhibition of the generation of said otherpulses.
 10. Apparatus according to claim 9, said slow down andstop-positioning mode operation continuing until said movable memberreaches a position in the proximity of the new position whereupon pulsesare provided for (a) counting said delta counter to zero; (b)incrementing said internal counter to have new contents representing thecommand position within one of said cycles as measured with respect toan adjacent sine zero; (c) changing the contents of said externalcounter to represent the command position with respect to a referencezero; and means for putting said motor under control of the error signalderived from said position-measuring device.
 11. Apparatus according toclaim 9, comprising means for storing the sign of the number in saiddelta counter, and means for displaying said last-mentioned sign andcertain least significant digits of said delta counter.
 12. Apparatusaccording to claim 8 wherein after positioning in a slow down and stopmode said external counter contains a count representing the actualposition said first member reaches in coming to rest at the desiredcommand position including undershoot or overshoot.
 13. Apparatusaccording to claim 12 including means for displaying the amount and signof saiD undershoot or overshoot.
 14. Machine tool control apparatuscomprising means for comparing the count in temporary storage with thecount in a first counter, means operative when said comparison indicatedinequality to (a) load a second reversible counter with certain pulsesand (b) direct said certain pulses to correspondingly change thecontents of said first counter, means for deriving said certain pulsesfrom other pulses derived from the error current of a position measuringdevice having a movable element fixed to a movable machine member drivenby a variable speed motor, a control device responsive to the count insaid second reversible counter to control the operation of said motor,means for decrementing both of said counters in response to said otherpulses, and means changing the frequency of said other pulses inresponse to the magnitude of said error current.